The application of digitally controlled inkjet technology to processes heretofore carried out by analog presses is becoming widespread. Applications including the printing of film-based packaging and labels have historically been performed by flexographic or rotogravure based analog printing presses. Such analog printing presses accept a roll of print media material which is unwound and passed through various processes including printing and curing of ink. Printing occurs between wetted drums having physically imaged surfaces and the web media. Each drum applies a single primary color sub-image and the ink is typically cured or set between the application of each different primary color. Such a process is very efficient for large batches of a given printed image but does require changing drums in order to print a new image.
Inkjet digital web presses offer an advantage of being able to change a printed design without changing a drum. Inkjet presses pass print media under “print bars” which have arrays of inkjet printheads that span the print zone. These print bars are under computer control and allow printed designs to be rapidly changed within a reel of material thus eliminating a new printer setup for a new printed image. Inkjet web presses are particularly advantageous for “short run” printing in which only a relatively small number of a particular print design are required. In fact, every print can be different—an example might be packaging with an embedded serial number that changes with each package.
Although shown to be advantageous for digital presses, inkjet printing has historically been optimized for home printers using small disposable cartridges. A big challenge for inkjet printing in digital presses is the need to properly deliver ink to the printheads. Typical inkjet printheads eject droplets of ink from very tiny nozzles that refill themselves via capillary action. For proper function of the nozzles and to avoid problems such as “drooling” the ink needs to be delivered to the printheads at a carefully controlled pressure. Typically this pressure is a slightly negative “gauge” pressure meaning that it is slightly below the pressure of the surrounding atmosphere. In some embodiments this pressure may be in a range from about −0.5 to about −3.0 inches of water although the optimal gauge pressure may vary according to various factors such as ink surface tension and nozzle size.
Systems for providing ink in this manner in digital presses have typically been quite complicated and have required high maintenance. Some have involved modulating the pressure using electrically controlled air pressure regulators as one example. While prior art systems have been made to work there is an ongoing need to provide simple designs that are inherently reliable and reduce maintenance and downtime costs.